The Rel signal transduction pathway is of fundamental importance in biological processes as diverse as Drosophila development and vertebrate immunity. This project is designed to study the function of zygotic cact, and specifically test if zygotic dl is the biological target of zygotic cact or whether it regulates the function of some other Rel protein. The specific goals of this study are: (1) To determine the spatial and temporal pattern of expression of the zygotic cactus protein in the late embryo as well as during larval stages. To establish its function, we will ask whether the expression of cactus is tissue restricted or if it is ubiquitously expressed, and whether this expression is coincidental with that of dl. (2) To define the structural requirements essential for optimal cactus function. The maternal and zygotic cact proteins are very similar in structure and the maternal target of cact is known. Molecular analysis of zygotic alleles of cact, when compared with that of the maternal effect alleles will shed light on the mechanism of specificity of interaction with dorsal or another zygotic partner. (3) To define and analyze new components that interact with zygotic cactus. Histochemical, genetic and molecular methods will be used to address these question. The functional analysis of cactus in other stages of development and tissues of drosophila will provide information about, and access to, key biological processes regulated by similar signal transduction systems that use the inhibitory functions of the zygotic cact. Since Drosophila is genetically accessible, it serves as an excellent system for a thorough functional analysis of proteins in vivo. Our studies on cact structure governs its function will have a direct bearing on the two central issues of signal transduction: mechanism and specificity.